AP Biology Unit 3 Topic 2: Cellular energy

What to Know

ATP stores usable chemical energy in phosphate bonds and can transfer energy through phosphorylation.
Catabolic pathways break molecules down and release energy, while anabolic pathways build molecules and require energy.
Energy transformations are never perfectly efficient, so heat and entropy changes matter in living systems.
Always connect this topic back to the larger unit: Cellular Energetics.

Detailed Notes

Cellular energy belongs to Unit 3: Cellular Energetics. Cells manage free energy by coupling energy-releasing reactions to energy-requiring processes. Treat the topic as a relationship between structure, process, evidence, and biological function instead of memorizing an isolated definition.

Key relationships: ATP stores usable chemical energy in phosphate bonds and can transfer energy through phosphorylation. Catabolic pathways break molecules down and release energy, while anabolic pathways build molecules and require energy. Energy transformations are never perfectly efficient, so heat and entropy changes matter in living systems.

Model to use: Trace energy from a food molecule to ATP and then to a cell process such as transport, movement, or biosynthesis. A clear model should label the parts, show the direction of movement or information flow when relevant, and connect the diagram back to the biological outcome.

AP-style move: When asked whether a process can happen, decide whether it releases energy, requires energy, or must be coupled to another reaction. Strong answers name the concept, apply it to the scenario, and explain the reasoning in complete sentences.

Key Vocabulary

Free energy

Energy available to do work in a biological or chemical system.

ATP hydrolysis

The breakdown of ATP that releases energy for cellular work.

Coupled reaction

A pairing of reactions in which an energy-releasing process powers an energy-requiring process.

Phosphorylation

Addition of a phosphate group to a molecule.

Catabolic pathway

A metabolic pathway that breaks molecules down and releases energy.

Anabolic pathway

A metabolic pathway that builds molecules and requires energy input.

Quick Practice

How would you explain Cellular energy in one or two AP-style sentences?

Name the concept, apply it to a specific example or source, and explain the reasoning that connects the evidence to your answer.

Official Alignment

Reference and Attribution

Biology concept reference: Biology for AP Courses by OpenStax, Rice University. Access for free at OpenStax. Licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License. EduCompanion used this source as a conceptual reference and rewrote the study notes in original wording.